1. Field of the Invention
The present invention is generally directed to the processing of electrical signals, and more particularly to the processing of ECG signals and the treatment of cardiac conditions in an ambulatory medical device, such as a wearable defibrillator.
2. Discussion of the Related Art
To protect against cardiac arrest and other cardiac health ailments, some at-risk patients may use a wearable defibrillator, such as the LifeVest® wearable cardioverter defibrillator available from ZOLL Medical Corporation of Chelmsford, Mass. To remain protected, the patient wears the device nearly continuously while going about their normal daily activities.
With an ambulatory medical device, such as a wearable defibrillator, the patient's electrocardiogram (ECG) signal is obtained from body surface electrodes. When the ECG signal is obtained in this manner, electrical noise frequently degrades the quality of the ECG signal. The challenge becomes one of extracting a clean ECG signal from the sometimes noisy signals derived from the body-surface electrodes. A typical ECG measurement system includes a signal acquisition circuit (also called an analog front end (AFE)) that amplifies the ECG signals gathered by the electrodes, an analog-to-digital converter (ADC) that digitizes the amplified ECG signals and a processor that analyzes the ECG signals and controls the ambulatory medical device based on the processed ECG signals.
The ECG signals provided by the electrodes are typically about 80 microvolts to about 2 millivolts in signal amplitude. The typical signal acquisition circuit amplifies the ECG signals received from the body surface electrodes by about 500 to 1000 times before providing the amplified ECG signal to the ADC to digitize the ECG signal. Unfortunately, any amplification of the ECG signal also amplifies any noise present in the ECG signal. To maximize the signal-to-noise ratio and reduce noise in the system, the amplification of the ECG signal is completed in multiple stages. These multiple stages typically include one or more amplifier or gain stages and a controlled or programmable attenuation stage. However, these multiple amplifier stages can make the system susceptible to analog and/or digital signal clipping. As defined herein, clipping is a form of signal distortion that cuts off or “clips” the signal once the gain of the amplifier exceeds a certain threshold or when the ADC is at its minimum or maximum voltage range. Typically, the threshold above which analog clipping occurs is the maximum output level of the amplifier.
Consequences of analog clipping can include distorted ECG signals presented to the ADC input. In addition, once analog clipping starts to occur, the ECG signal being processed by the signal acquisition circuit will no longer respond to the control signals sent by the system and can cause the signal acquisition system to malfunction.